Modern communication or transmit systems are based on digital signal processing technology. Such communication systems can transmit and receive signals. Digital communication techniques are used in almost all areas of wireless communication such as radio communication, telecommunication and local area networks. Digital communication is based on a special class of communication signals, sent by a transmitter and received by a receiver, using digital modulation techniques. These techniques make use of a class of signal modulators, which perform vector or quadrature modulation. A vector-modulated or quadrature-modulated signal is an amplitude and phase modulated signal that is modulated according to in-phase and quadrature-phase components, which define a phase vector. In vector or quadrature modulation, two modulation input signals independently modulate the in-phase component or signal (I) and the quadrature-phase component or signal (Q) of a carrier signal. A usual digital transmitter architecture comprises an up-conversion of the transmit signal from baseband to the RF carrier frequency by a tunable or fixed analog I/Q modulator (=I/Q up-conversion).
However, unavoidable imbalances between I- and Q-branches in the analog part of the transmitter lead to an insufficient suppression of the image signal. Due to hardware tolerances a perfectly balanced analog I/Q modulator is not feasible, resulting in a limited suppression of the image signal. The main issues in designing a I/Q modulator are compensation of the I/Q imbalance by maintenance of quadrature phase between I and Q channels (i.e. minimization of amplitude and/or phase errors between I and Q channels) and minimization of direct current (DC) offsets (i.e. minimization of carrier leakage). Each of these factors can introduce distortion into the transmitted signal which can result in an increased bit-error rate at the receiver. Furthermore, these factors vary with temperature, device biasing, component aging and frequency, often making readjustment during operation necessary. Therefore the effects of I/Q imbalance must be compensated, e.g. by using digital signal processing.
Digital compensation of analog I/Q imbalance has been widely described in different documents. Most publications consider I/Q imbalances at the receiver. Because here the analog imbalances occur before the digital part, a straight-forward compensation technique based on adaptive filtering or blind coefficient estimation can be realized.
Other systems use compensation of I/Q imbalances at the transmitter. Such methods are much more difficult. The U.S. Pat. No. 6,298,096 B1 describe a transmit modulator which uses a quadrature modulator having a predistortion block which produces a pre-distorted output signal. The predistortion block pre-compensates errors introduced by the quadrature modulator based on a set of predistortion coefficients. The quadrature modulator receives the output signals from the predistortion block. The quadrature modulator up-converts the I and Q channel signals and combines them. In the process, the quadrature modulator introduces errors. In calibration mode, a sinusoidal wave at a calibration frequency is applied to the input of the predistortion block and a transformer is coupled to the output of the quadrature modulator. The transformer produces a digital representation of a spectrum of the output of the quadrature modulator converted to baseband. Spurious energy produced by the quadrature modulator errors but reduced by the effect of the predistortion block is generated at the calibration frequency and the double of the calibration frequency. A quadric polynomial minimization calculator receives the output of the transformer. The quadratic polynomial minimization calculator determines a subsequent value of the predistortion coefficients based upon a quadratic relationship between energies present in the digital representation of the spectrum of the output of the quadrature modulator at the calibration frequencies and the values of the previous predistortion coefficients. This prior art compensation method bases on re-executed iterative process. After an initial determination has been made, each time the iterative compensation process is re-executed, the previous values of the predistortion coefficients are used as a starting point. This process requires extra time to converge upon optimal values.
Furthermore, the EP 04106643.2 describes another method for LO leakage and I/Q mismatch calibration. This calibration method bases on the adjustment of a calibration signal by at least one predetermined compensation coefficient. This compensation coefficient is determined in three calibration steps. This process requires also extra time to converge upon optimal compensation values.